1. Field of the Invention
This invention relates to a heat storage material which consists of a mixture of dimethyl terephthalate and a compound selected from the group consisting of dimethyl fumarate and dihydroanthracene. More particularly, the invention relates to a heat storage material which is suitable for use in devices which utilize, for example, solar energy for room heating or cooling, hot-water supply and so forth.
2. Description of the Prior Art
The characteristic properties that are generally required of heat storage materials are large specific heat and/or large latent heat of fusion, thermal stability, noncorrosiveness, low vapor pressure, noninflammability and innoxiousness.
When the latent heat of fusion is utilized for storing thermal energy, the amount of stored heat can be made large and the volume of the heat storage material can be made small, which are quite advantageous in practice. A material which is suitable for storing thermal energy by its latent heat of fusion is characteristically converted from one phase to another phase when it is heated to its own phase transition temperature. Various materials have been proposed as heat storage materials with the phase transition, for example, inorganic salt hydrates such as calcium chloride hexahydrate (U.S. Pat. No. 4,189,394--J. Schroder et al.), magnesium iodide hexahydrate, sodium sulfate decahydrate, barium hydroxide octahydrate and ammonium aluminum sulfate dodecahydrate; organic compounds or mixtures thereof such as tetradecane, pentadecane, decanol and a mixture of sodium acetate and sodium chloride; and low molecular weight organic salts. When inorganic salts or low molecular weight organic salts are used, however, the problem of corrosion of the thermal system becomes serious. Therefore, the tanks, pipes and other devices of the thermal system are generally made of noncorroding metals so that they are usually heavy and high in thermal conductivity. In addition, they are generally expensive, which increases the construction cost of the thermal energy storage system. Furthermore, there always remains the fear that the tanks or pipes may break, resulting in leakage of solution or fused salts.
If an inorganic salt is mixed with another inorganic salt, a composition having a desired melting point can be obtained. However, if the composition deviates from the eutectic mixture of the inorganic salt components, separation of components occurs during solidification of the fused liquid. Accordingly, only eutectic mixtures are used as the heat storage materials. However, eutectic mixtures of inorganic salts are liable to supercool to temperatures considerably below the eutectic points. Therefore, it is necessary to add seed crystals or a nucleating agent, which causes the above-mentioned separation of components. Incidentally, when a paraffin mixture is employed as the heat storage material, it is not advantageous in practice because the range of melting point is wide and the production cost for pure paraffin is quite high. Furthermore, there are proposed mixtures containing higher fatty acids such as lauric acid, stearic acid and oleic acid (U.S. Pat. Nos. 2,726,211--V.J. Schaefer and 4,100,092--H.O. Spauschus et al., and British Pat. No. 1,558,522--Ciba-Geigy AG). However, the mixtures do not meet the later-described requirement of 90.degree. to 100.degree. C. in melting point.
With the increasing concern over the depletion of ordinary energy sources, considerable attention has been given to the use of solar energy, and various kinds of systems utilizing solar energy are being developed. Solar energy water heaters have already been used in practice. With them, water is heated by solar heat, and the obtained hot water is used as it stands or after being temperature-controlled, for room heating, bathing, cooking and washing. Besides the use of solar energy for heating and cooking with hot water, a new system is now being eagerly developed to drive a Rankine cycle engine by using the obtained hot water for the purpose of cooling. In order to drive the Rankine cycle engine of this system by using water as a heat transfer medium, it is desirable that the water temperature be as high as possible, about 90.degree. to 100.degree. C. Therefore, the key to accomplishing the above system is to develop an improved thermal energy storage material which can store much solar energy at high temperature and which can supply a large quantity of high temperature water. In other words, the heat storage material which is earnestly desired in this technical field is one which is free from the above-described difficulties, which is produced at low cost, and which has a large heat of fusion and a proper melting point in the range of 90.degree. to 100.degree. C.